Wind turbines for converting wind energy to electrical power have been known and applied for many years, but have found a dramatically increased application as an alternative energy source during the last couple of decades. It has become common to place wind turbines together in large groups, often counting in the hundreds, within a restricted area. Such large collections of wind turbines can provide an environmentally less desirable solution both from an aesthetic point of view and also due to the inevitable noise issues they may raise. Furthermore, the positioning of wind turbines on land may not always be an optimal placement, as it is preferable that the blades of the wind turbine be located in a laminar flow of air, which is not always obtained on land due to, for instance, the presence of hills, woods, buildings, etc. It has hence become popular to locate groups of wind turbines offshore, not too far from the coast at locations where water depths allow the wind turbines to be fixedly attached to a foundation floating in the sea or extending to the bottom of the sea. Over water, the flow of air is not disturbed by the presence of various obstacles as mentioned above and furthermore, such placements may be advantageous from an environmental point of view.
A wind turbine tower is generally 50-100 m in height with weights approaching 100 tons or more for a turbine with a 90 m rotor diameter (e.g., 2 MW unit). Due to the large dimensions of present day wind turbines, dimensions which furthermore tend to increase due to the relation between the diameter of the rotor and the maximum electrical power which the wind turbine can provide; the forces formed by rotational movement of the wind turbine blades; and the very large surface of the tower being exposed to the wind, it is vitally important that the wind turbine be provided with a stable foundation. This is not in principle a problem for wind turbines located onshore, but becomes more problematic in connection with offshore wind turbine installations.
For relatively shallow water depths (e.g., up to about 25 m), one solution is to mount the tower of the wind turbine on a suitable construction of pillows and grids, which are in turn fixed to a firm foundation on the bottom of the sea. For applications having increased water depths, the options become more limited. By way of example, for medium water depths of up to about 100 m, various floating foundations have been utilized to support the wind turbine. In this regard, WO 01/73292 discloses a floating offshore wind power installation. Such floating foundations, however, may be relatively complex in their design, expensive to construct and assemble, and difficult to install in the sea.
Alternative designs, however, may include pile foundations or gravity-based foundations. In each of these cases, the foundation is typically formed as a one-piece structure, such as quayside, and then subsequently transported to the offshore installation site and positioned within the sea. While this may be acceptable for shallow water depths, for medium water depths, the foundations are extremely large in size (e.g., over 100 m in length) and the transport and installation of such large structures is complex and costly. By way of example, to accommodate the sheer size of the one-piece foundations, heavy lift crane vessels are typically used to unload the foundation and locate it on the seabed. Additionally, it may be difficult to control and guide the large, one-piece foundation as it is being lowered toward the seabed. Furthermore, these foundations may utilize ballasts or other devices to help sink and stabilize the foundation and may also require costly preparation of the seabed prior to installation.
Accordingly, there is a need for an improved offshore foundation which can be transported to an offshore installation site, assembled, and installed on the seabed in a less costly, more efficient manner.